1. Field of the Invention
This invention relates to resinous particles, a method for the production thereof, and uses therefor. More particularly, in the suspension polymerization of a polymeric monomer having as a main component thereof a (meth)acryl type monomer, this invention relates to technique for repressing the sympathetic occurrence of minute particles due to emulsion polymerization, stabilizing the suspension polymerization, enhancing the yield of the polymerization, and enabling the produced resinous particles to acquire enhanced physical properties. Further, this invention relates to a technique for improving the ability of (meth)acryl type resinous particles to resist heat.
2. Prior Art
(Meth)acryl type resinous particles particularly of the class having an average particle diameter in the approximate range of 0.1 to 500 .mu.m are expected to find utility in numerous applications such as for example as an anti-blocking agent for resinous film, an additive for electrostatic image developing toner, a powder coating material and water-dispersion type coating material, an additive for facing panels, an additive for artificial marble, a filler for cosmetic articles, and a filler for a chromatographic column.
Heretofore, as methods for the production of resinous particles, those resorting to mechanical pulverization, to suspension polymerization, and to emulsion polymerization have been known to the art. Of these methods, that which resorts to mechanical pulverization necessitates a huge energy input to pulverize the particles and require many classification steps to enable the resinous particles to be produced with a uniform diameter. Since the minute particles which are obtained by this method are amorphous morphologically, they have room for further improvement in flowability and proofness against flocculation. Although the method which resorts to emulsion polymerization is capable of producing minute particles of uniform diameter, the produced minute particles have a diameter of about 0.1 .mu.m and, therefore, cannot be directly put to such applications as mentioned above. In contrast thereto, the method which resorts to suspension polymerization allows relatively easy production of resinous particles of a desired diameter because it comprises preparing suspended particles of a monomer by mechanical stirring and subjecting the suspended monomer particles to polymerization. It further enjoys such advantages as obviating the necessity for using a solvent and facilitating the reaction control.
It has been known, however, that the suspension polymerization entails secondary formation in the aqueous phase of minute particles due to emulsion polymerization. The secondary reaction lowers the yield of the main polymerization and degrades the stability of this polymerization. Further, since the minute particles formed as described above by emulsion polymerization adhere predominantly to the phase boundary of the particles produced by suspension polymerization and cannot easily be completely removed therefrom , they suffer impairment of the physical properties of the produced resinous particles. Particularly when the resinous particles to be obtained by suspension polymerization are required to have such a small diameter as to be in the approximate range of 0.1 to 500 .mu.m, for example, and since the amount of a dispersion stabilizer to be added to the aqueous phase for ensuring stabilization of the minute suspended particles is large as compared with that for ordinary suspension polymerization, the amount of the polymeric monomer dissolved in the aqueous phase at the step of dispersion and the step of polymerization is increased possibly to the extent of causing the problem of by-production of minute particles due to emulsion polymerization.
As a means for preventing the suspension polymerization from causing emulsion polymerization in the aqueous phase, the addition of an inorganic water-soluble inhibitor to the system has been known to the art. For example, JP-A-55-83,125 discloses the addition of 0.01 to 10% by weight of a water-soluble inhibitor such as ammonium thiocyanate or cupric chloride to water, JP-A-60-8,302 discloses the addition of vanadium pentoxide and/or cupric chloride in combination with a dispersion stabilizer, JP-A-62-205,108 discloses the solution in water of not less than 10 ppm, based on the total amount of vinyl monomers, of such a water-soluble inhibitor as sodium nitrite, potassium nitrite, or cupric chloride, JP-A-2-284,905 discloses suspension polymerization effected by the use of a water-soluble inhibitor such as a nitrite and a polymerization initiator formed of an organic peroxide, and JP-A-3-237,105 discloses suspension polymerization effected in a continuous aqueous phase containing water, a water-miscible organic solvent, and a water-soluble polymerization inhibitor such as sodium nitride or hydroquinone.
Further, as disclosed in JP-A-61-255,353, the technique of adding to an aqueous suspension polymerization system a water-soluble mercaptan compound for the prevention of the sympathetic occurrence of emulsion polymerization has been known to the art. As water-soluble mercaptan compounds, 2-mercaptoethanol, thioglycolic acid, cysteine, glutathione, dimercaprol, 1,4-dithiothreitol, dimercaptosuccinic acid, and 2,3-dimercapto-1-propanesulfonic acid are cited in the specification in support of the disclosure.
JP-A-52-102,391 discloses the addition of about 0.0005 to about 0.02 part by weight of a water-soluble inhibitor selected from among borohydrides represented by the following structural formula (I), alkali metal nitrites, alkaline earth metal nitrites, and ammonium nitrite and about 0.0001 to about 0.005 parts by weight of an oil-soluble inhibitor, oil-soluble and alcohol-soluble nigrosine, respectively based on 100 parts by weight of monomer. ##STR1## (wherein X is an alkali metal and R, R', and R" independently represent a hydrogen atom, a phenyl group, an alkoxy group, or an alkyl group of one to ten carbon atoms).
In the suspension polymerization of a polymeric monomer having as a main component thereof a (meth)acryl type monomer, even when an inorganic water-soluble inhibitor is added to the reaction system, the effect of the inhibitor in preventing the emulsion polymerization is so weak that the inhibitor must be added in a large amount. Particularly, the conspicuity of this trend grows in proportion as the diameter of minute particles obtained by the suspension polymerization decreases.
When the water-soluble mercaptan compound mentioned above is used for preventing the sympathetic occurrence of emulsion polymerization, this mercaptan compound induces impartation of an offensive odor to the produced resinous particles or the effluent from the polymerization system. This offensive odor is not easily removed by washing.
As regards the use of the borohydride as a water-soluble inhibitor, this compound itself is difficult to handle and, on account of this difficulty, the conditions of the suspension polymerization dictate rigid control.
The various applications cited above are possible outlets for the (meth)acryl type resinous particles. When the resinous particles are used for example, as an anti-blocking agent for resinous film, the characteristic properties which the resinous particles are desirably possess are proximity in the refractive index to the system of resin to which the agent is to be added and ideally the ability to withstand heat.
For example, polyolefin film, is used as a packaging material for wrapping various articles of commerce such as foodstuffs. The film is at a disadvantage in respect that when it is superposed over itself in a multiplicity of layers, the adjoining surfaces of the superposed layers of film fasten cohesively a phenomenon popularly called "blocking," and seriously impairs the operational efficiency of the wrapping. As means conventionally adopted for preventing the polyolefin film from the phenomenon of blocking and imparting an improved slip property to the film, a method which comprises uniformly incorporating minute particles of an inorganic substance such as silica or talc in the film has become very polular. For the purpose of imparting sufficiently the anti-blocking property and slip property to the polyolefin film by the use of such an inorganic substance as mentioned above, however, the inorganic substance must be incorporated in a large amount into the film. Further, when the polyolefin film containing the inorganic substance is stretched., the film is at a disadvantage in that voids occur round the peripheries of the particles of inorganic substance which decreases the film transparency and mechanical strength.
The use of fine powder of polyamide or beads of condensed resin having a triazine ring instead of the inorganic substance mentioned above has been proposed. A method which resorts to the use of the fine powder of polyamide is effective in preventing degradation of transparency. However, it, has the disadvantage in that since it requires this fine powder to be used in a large amount, the fine powder degrades the strength of the film and boosts the cost of production of the film. A method which relies on the use of beads of condensed resin having a triazine ring is unable to adequately prevent the impairment of transparency because of the difference in refractive index between the polyolefin and the condensed resin having a triazine ring. It is further at a disadvantage in that the unaltered volatile substances such as formalin which persist in the condensed resin having a triazine ring give rise to voids during stretching and bring about degradation of the transparency or mechanical strength. Further, at the time that these resinous particles are incorporated in a film, the resinous particles decompose during melting and kneading of the resin for film unless they have sufficient heat stability and the product of this decomposition emits an offensive odor and induces the occurrence of voids possibly to the extent of polluting the environment and impairing mechanical and optical properties of the produced film. Thus, the resinous particles should desirably have a further improved heat stability.